Windows 7 Family Pack Question

I noticed that Newegg.com now has the Windows 7 Family Pack Upgrade 3 pack for $123.99 with free shipping and was thinking about purchasing it. I have 2 legit Vista systems that I would like to use it on to upgrade to 7.

I also have a desktop PC running a not-so-legit version of Windows XP Pro. Will the upgrade work with it? If not, any suggestions on how to get it to work?